The present invention relates to signal processing apparatus for processing relatively weak electrical signals particularly to increase the signal-to-noise ratio. The invention is especially useful in electrocardiograph equipment, and is therefore described below with respect to this application.
ECG'S (electrocardiograms) are electrical potential traces accompanied by the contractions of the different cavities of the heart. They are an important aid in the study and diagnosis of abnormal heart activity. Particularly important in this respect is the His-Purkinje ECG (HPE) activity which, up to about ten years ago, could be recorded only by an invasive procedure in which an electrode was inserted into the heart by means of a catheter. In recent years, however, capabilities have been developed for detecting and recording HPE activity from the body surface by non-invasive procedures, but the techniques used for this purpose generally involve elaborate and expensive recording equipment which is not readily available at the clinical level. Therefore, non-invasive monitoring of HPE activity is generally limited to a few laboratories and has not become a standard cardiac test. Other cardiac activities extremely difficult or impossible to monitor by non-invasive techniques with present equipment include the discernment of the P-wave subcomponents which may denote various supraventricular structures, and the discernment of after-potentials which may herald ventricular fibrillation.
The previously known non-invasive systems for processing ECG signals particularly to monitor HPE activity generally use the well known signal averaging technique for enhancing the signal and reducing noise. Signal averaging increases the signal-to-noise ratio of repetitively occuring data by serially adding together the repetitive signals, each addition being initiated by a synchronizing signal providing a reference or fiducial point having a constant time relation to the data signal. Thus, the coherent portions of the signal (data) reinforce each other with each successive addition, whereas the non-coherent portions (noise) add out-of-phase and therefore tend to cancel out with each successive addition. The quality of the synchronizing or triggering point for each addition cycle is critical to the quality of the averaging process, since any variation in time between the signal of interest and the fiducial or reference point, referred to as jitter, will rapidly degrade the produced average signal.